1. Field of Invention
This invention pertains generally to a method for investigation of subterranean geological formations with circularly polarized energy and more particularly to the determination of anisotropy using circularly polarized energy.
2. Description of Prior Art
It is well known in the geological and geophysical arts that cracks and fissures in the rock strata create anisotropic conditions for acoustical energy propagation. Attempts have been made to exploit the presence and orientation of these conditions inasmuch as the geological features which create them indicate a high likelihood for the presence of hydrocarbons. These previous attempts have all utilized the wave splitting characteristics exhibited by these features when exposed to linearly polarized energy. This invention employs circularly, as opposed to linearly, polarized energy to obviate many of the major disadvantages accompanying the use of linearly polarized energy while yielding improved results in the exploration of anisotropic geological formations.
The prior art teaches that when a linearly polarized wave ideally encounters an anisotropic condition it is split into two linear components that are polarized along the two axes of the anisotropic feature orthogonal to direction of propagation of the source wave. The anisotropy will then attenuate the amplitude and slow the propagation of the component which is polarized normal to the anisotropic feature. The components are eventually reflected to receivers located at the surface or suspended in a borehole whereupon the reflected arrivals are recorded and analyzed. The traces representing the reflected arrivals are analyzed for differences in amplitude and propagation velocity using, for example, particle motion ellipticity to determine the presence and orientation of anisotropic features in the geological formation.
Typical of such approaches in the prior art are U.S. Pat. No. 4,789,969, to Naville, U.S. Pat. No. 4,817,061, to Alford et al., the publication "Detection of Anisotropy Using Shear Wave Splitting and VSP Surveys: Requirements and Applications", by Charles Naville appearing in Expanded Abstracts with Biographies for the 56th SEG Meeting, pps. 391-394, and the publication "Practical Use of Shear Wave Splitting in VSP Surveys", by Charles Naville.
The translation of the theory into practice is naturally fraught with many complications. One major drawback to the practice of the prior art is that surface waves and random noise generate ellipticity in the data and therefore error in this method of analysis. Another major drawback occurs when the direction of propagation of the source wave is oriented too closely along one of the principal axes of the anisotropic feature, which prevents wave splitting so that there is no ellipticity in the data even though anisotropic conditions exist. The presence or absence of ellipticity therefore does not necessarily accurately indicate anisotropic conditions or the orientation of the anisotropic axes.
It is therefore a feature of the invention to provide a better way to minimize the interference of surface waves and random noise in the detection of anisotropic conditions in geological formations.
It is a further feature of the invention to provide a better method of ascertaining the presence of anisotropic conditions and the orientation of the principle axes of anisotropy by detecting such conditions regardless of the angle of propagation of the source wave with respect to the principal axes of anisotropy.